This invention relates to ink jet printing apparatus and more particularly to a control mechanism for maintaining a substantially constant spatial relationship between one or more ink jet print heads and print-receiving surfaces, such as on one or opposite sides of an object moving past the print heads.
The control mechanism of this invention is adapted to be incorporated into an ink jet printing apparatus of the kind that can be operated to print indicia onto a substrate. The indicia can be characters, symbols and, in more recent developments of the art, bar codes. Such ink jet printing apparatus generally includes- one or more print-heads.
Each print head has a vertical array of orifice nozzles from which ink can be emitted under pressure in the form of droplets emerging from the print head for impact upon the substrate. The nozzles are connected through individual valves to an ink source that is maintained under pressure. A programmable controller regulates the operation of the valves to cause ink to flow through them to and to be emitted from the nozzles according to a pre-selected pattern or patterns. In this way, the ink droplets form the desired symbol, character, or bar codes on the substrate.
When ink is emitted from an ink jet nozzle, it must travel a small fraction of an inch to form an ink droplet. This ink droplet grows wider as the distance of travel from the nozzle increases. As the width of the droplet increases, a larger dot will be printed upon a substrate impacted by the droplet. As a result, as the droplet gets wider, the outer edges of the printed dot lose precision, and the quality of the printing deteriorates.
In a typical installation, the substrate moves relative to the print head or print heads. For example, the substrate may be defined as one or more surfaces on a package such as a carton or container, and there may be a plurality of such packages in a row on a conveyor that transports the packages successively past the print head or print heads. It is common to have one or more print heads on opposite sides of the conveyor so that printing can be done on opposite faces of the package simultaneously.
To guide the packages into position relative to the print head or print heads, guide rails have been provided on opposite sides of the conveyor. These guide rails define planes that are spaced a predetermined distance from the print head nozzles, and the guide rails hold the packages between them as they are transported by the conveyor. However, packages of the same nominal size vary in width due to manufacturing and friction tolerances and there are undulations in the faces of the packages. Therefore, the span between guide rails must be great enough to accommodate the widest of the packages as permitted by the size tolerances. Accordingly, although the guide rails hold the packages at nominal distances from the print head nozzles, those distances vary as different package surfaces pass the nozzles and are imprinted.
Printing quality has always been important in the ink jet printing art. Efforts have been made to improve the quality of ink jet printing, resulting in improvements in the design of the nozzles, such as by the incorporation of jewel led orifice nozzles, and resulting in improvements in the composition of the inks. Examples of the results of some of these efforts are set forth in U.S. Pat. No. 4,378,564. As a result of these kinds of efforts and developments, the quality of ink jet printing has improved greatly, and ink jet printing has succeeded in meeting the requirements of a growing number of diverse applications.
In all of these applications to date, an optimum range of spacing between the ink jet nozzles on the print head and the substrate has existed and print quality has been limited by the tolerance in this spacing that has accommodated the variations in sizes of the packages. However, as the art continues to develop, the desire and demand for even increased printing quality have increased. Moreover, with the introduction of machine readable ink jet printing, such as the printing of bar codes, the need for more precise printing has emerged.